Minimization of bias temperature instability (bti) degradation in circuits

ABSTRACT

A circuit structure is provided. The circuit structure includes first pfet device. The circuit structure further includes a first nfet device connected to the pfet device. The circuit structure further includes a keeper nfet device that reduces stress associated with the first nfet device by keeping the first nfet device off during its functional state. The circuit structure further includes a keeper pfet device that reduces stress associated with the first pfet device by keeping the first pfet device off during its functional state.

FIELD OF INVENTION

The present invention relates to integrated circuits, and more particularly to minimization of bias temperature instability (BTI) degradation in integrated circuits.

BACKGROUND

Reliability issues in very large scale integrated (VLSI) integrated circuits have been a growing concern, as technology trends in semiconductor technologies continues to grow and, show progressive downscaling of feature sizes, of the large scale integrated circuits. As an example, one major reliability issue of VLSI is bias temperature instability (BTI), which causes threshold voltage, Vth, of CMOS transistors of the VLSI, to increase over time, under voltage stress, resulting in a temporally-dependent degradation of digital logic circuit delay of VLSI. BTI is also a dominant reliability concern for nano-scale PFET (NBTI) and NFET (PBTI) transistors of VLSI. NBTI (Negative Bias temperature instability) occurs under negative gate voltage (e.g., Vgs=−VDD), and is measured as an increase in the magnitude of threshold voltage. This, consequently, affects P-type metal-oxide-semiconductor (PMOS) transistor, and degrades device drive current, circuit speed, noise margin, and the matching property of the integrated circuit.

SUMMARY

According to one embodiment, a circuit structure is provided for minimizing bias temperature instability (BTI) degradation of the circuit structure, is provided. The circuit structure includes a first pfet device. The circuit structure further includes a first nfet device connected to the pfet device. The circuit structure further includes a keeper nfet device that reduces stress associated with the first nfet device by keeping the first nfet device off during its functional state. The circuit structure further includes a keeper pfet device that reduces stress associated with the first pfet device by keeping the first pfet device off during its functional state.

According to another embodiment, a method for minimizing bias temperature instability (BTI) degradation of an integrated circuit, is provided. The method includes providing a first pfet device. The method further includes providing a first nfet device connected to the pfet device. The method further includes providing a keeper nfet device for reducing stress associated with the first nfet device by keeping the first nfet device off during its functional state. The method further includes providing a keeper pfet device for reducing stress associated with the first pfet device by keeping the first pfet device off during its functional state.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The drawings are not necessarily to scale. The drawings are merely schematic representations, not intended to portray specific parameters of the invention. The drawings are intended to depict only typical embodiments of the invention. In the drawings, like numbering represents like elements.

FIG. 1, is an integrated circuit (IC) for affected circuit stress due to bias temperature instability (BTI) degradation of the IC.

FIG. 2A illustrates a BTI circuit for affected circuit stress due to BTI degradation events of the IC FIG. 1, whereby, a NFET device of the IC, is stressed, during idle state of the IC, according to embodiments.

FIG. 2B illustrates a BTI circuit of the IC of FIG. 1, for affected circuit stress BTI degradation events of the IC of FIG. 1, whereby, VT stress is reduced on functional NFET, NFET is OFF(VGS=0), during IDLE state of the IC and, keeper NFET is turned ON, according to embodiments.

FIG. 3A illustrates a BTI circuit of FIG. 1 for affected circuit stress BTI degradation events of the IC of FIG. 1, whereby, PFET device of IC is stressed, during idle state of the IC, according to embodiments.

FIG. 3B illustrates a BTI circuit of the IC of FIG. 1, for affected circuit stress BTI degradation events of the IC of FIG. 1, whereby, VT stress is reduced on functional PFET by keeping the IC OFF(VGS=1), during IDLE state of the IC, and keeping keeper PFET ON, according to embodiments.

DETAILED DESCRIPTION

Detailed embodiments of the claimed structures and methods are disclosed herein; however, it may be understood that the disclosed embodiments are merely illustrative of the claimed structures and methods that may be embodied in various forms. This invention may, however, be embodied in many different forms and should not be construed as limited to the exemplary embodiments set forth herein. Rather, these exemplary embodiments are provided so that this disclosure will be thorough and complete and will fully convey the scope of this invention to those skilled in the art. In the description, details of well-known features and techniques may be omitted to avoid unnecessarily obscuring the presented embodiments.

References in the specification to “one embodiment”, “an embodiment”, “an example embodiment”, etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.

For purposes of the description hereinafter, the terms “upper”, “lower”, “right”, “left”, “vertical”, “horizontal”, “top”, “bottom”, and derivatives thereof shall relate to the disclosed structures and methods, as oriented in the drawing figures. The terms “overlying”, “atop”, “on top”, “positioned on” or “positioned atop” mean that a first element, such as a first structure, is present on a second element, such as a second structure, wherein intervening elements, such as an interface structure may be present between the first element and the second element. The term “direct contact” means that a first element, such as a first structure, and a second element, such as a second structure, are connected without any intermediary conducting, insulating or semiconductor layers at the interface of the two elements. In the interest of not obscuring the presentation of embodiments of the present invention, in the following detailed description, some processing steps or operations that are known in the art may have been combined together for presentation and for illustration purposes and in some instances may have not been described in detail. In other instances, some processing steps or operations that are known in the art may not be described at all. It should be understood that the following description is rather focused on the distinctive features or elements of various embodiments of the present invention.

As previously described, bias temperature instability (BTI) is a dominant reliability concern for nano-scale PFET (NBTI) and NFET (PBTI) transistors of VLSI. NBTI (Negative Bias temperature instability) occurs under negative gate voltage (e.g., Vgs=−VDD) of integrated circuits, and may be measured as an increase in the magnitude of threshold voltage of the integrated circuits. The NFET device exhibits an increase in non-mobile negative charge during symmetric (source=drain) stress. This shift results in larger magnitude threshold voltages over time (|ID| decreases). The PFET device exhibits an increase in non-mobile positive charge during symmetric (source=drain) stress. This shift results in larger magnitude threshold voltages over time (|ID| decreases). For example, for a matched PFET pair, NBTI can introduce a mismatch shift in several ways. If the gate biases or the equivalent time of the gate biases are not equal in the two PFET devices, this inequality will induce a systematic mismatch in Vt that can be calculated using the NBTI models given in PFET Negative-Bias Temperature Instability (NBTI). Further, process variations across the chip, such as tox, will cause systematic mismatch shifts between widely spaced devices. For closely spaced devices with equal gate biases, NBTI will induce a random mismatch shift in Vt and K such that the mismatch shift mean value is zero, but the s value increases with time

Embodiments of the present invention relates to integrated circuits, and more particularly to minimization of bias temperature instability (BTI) degradation of integrated circuits. The embodiments include one or more circuits, or subassemblies of circuits, as well as, a system, computer-implemented methods of operation, or circuit techniques and structure for reducing gate-source voltage of stressed devices, such as reliability phenomena in semiconductor devices e.g. NBTI, PBTI during idle state of the device, thereby putting the device in a relaxation mode. For example, the embodiments include an integrated circuit structure, which includes a first p-channel field-effect transistor (pfet) device, a first n-channel field-effect transistor (nfet) device connected to the device, a keeper n-channel field-effect transistor (nfet) device that reduces stress associated with the first nfet device by keeping the first nfet device off during its functional state, and a keeper pfet device that reduces stress associated with the first pfet device by keeping the first pfet device off during its functional state. The present invention addresses idle state BTI degradation of integrated circuits, surgically and, can be used for critical timing path of the integrated circuits. This will keep the circuit performance of the integrated circuits intact, through burin and life thus improving the product's effective performance significantly or allowing a longer lifetime.

Also, as will be appreciated by one skilled in the art, aspects of the present invention may be embodied as a system, method or computer program product. Accordingly, aspects of the present invention may take the form of an entirely hardware embodiment, an entirely software embodiment (including firmware, resident software, micro-code, etc.) or an embodiment combining software and hardware aspects that may all generally be referred to herein as a “circuit,” “module” or “system.” Furthermore, aspects of the present invention may take the form of a computer program product embodied in one or more computer readable medium(s) having computer readable program code embodied thereon. Any combination of one or more computer readable medium(s) may be utilized. The computer readable medium may be a computer readable signal medium or a computer readable storage medium.

A computer readable storage medium may be, for example, but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device, or any suitable combination of the foregoing. More specific examples (a non-exhaustive list) of the computer readable storage medium would include the following: an electrical connection having one or more wires, a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), an optical fiber, a portable compact disc read-only memory (CD-ROM), an optical storage device, a magnetic storage device, or any suitable combination of the foregoing.

In the context of this document, a computer readable storage medium may be any tangible medium that can contain, or store a program for use by or in connection with an instruction execution system, apparatus, or device. A computer readable signal medium may include a propagated data signal with computer readable program code embodied therein, for example, in baseband or as part of a carrier wave. Such a propagated signal may take any of a variety of forms, including, but not limited to, electro-magnetic, optical, or any suitable combination thereof.

A computer readable signal medium may be any computer readable medium that is not a computer readable storage medium and that can communicate, propagate, or transport a program for use by or in connection with an instruction execution system, apparatus, or device. Program code embodied on a computer readable medium may be transmitted using any appropriate medium, including but not limited to wireless, wireline, optical fiber cable, RF, etc., or any suitable combination of the foregoing.

Computer program code for carrying out operations for aspects of the present invention may be written in any combination of one or more programming languages, including an object oriented programming language such as JAVA, Smalltalk, C++ or the like, conventional procedural programming languages such as the “C” programming language, a hardware description language such as VERILOG, or similar programming languages. The program code may execute entirely on the user's computer, partly on the user's computer, as a stand-alone software package, partly on the user's computer and partly on a remote computer or entirely on the remote computer or server.

In the latter scenario, the remote computer may be connected to the user's computer through any type of network, including a local area network (LAN) or a wide area network (WAN), or the connection may be made to an external computer (for example, through the Internet using an Internet Service Provider). Aspects of the present invention are described below with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems) and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks.

These computer program instructions may also be stored in a computer readable medium that can direct a computer, other programmable data processing apparatus, or other devices to function in a particular manner, such that the instructions stored in the computer readable medium produce an article of manufacture including instructions which implement the function/act specified in the flowchart and/or block diagram block or blocks. The computer program instructions may also be loaded onto a computer, other programmable data processing apparatus, or other devices to cause a series of operational steps to be performed on the computer, other programmable apparatus or other devices to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide processes for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks.

Further aspects of the present invention will now be described in conjunction with the figures. Referring now to FIG. 1, an integrated circuit 100 for affected circuit stress due to bias temperature instability (BTI) degradation of integrated circuit 100, according to embodiments. Integrated circuit (IC) 100 is a set of circuits and subassemblies which can be susceptible to effects caused BTI degradation events.

IC 100 includes central processing unit (CPU) 110 and BIT circuit 120. CPU 110 executes program instructions on IC 100. BTI circuit 120 comprises a BTI structure that includes sensitive circuit elements for reducing gate-source voltage of stressed devices, such as reliability phenomena in semiconductor devices e.g. NBTI, PBTI during idle state of the device, thereby putting the device in a relaxation mode. IC 100 may include additional circuits or components, not depicted, for affected circuit stress BTI degradation of IC 100, according to embodiments.

FIG. 2A illustrates BTI circuit 120 for affected circuit stress due to BTI degradation events of IC 100 of FIG. 1, whereby, a NFET device of IC 100, is stressed, during idle state of IC 100, according to embodiments.

In the illustrated embodiment, pulldown transistor, i.e. NFET, of the BTI circuit 120 is stressed during the idle time. More particularly, as depicted, a tabular capture of the states of stress and relaxation when the IC 100 of FIG. 1 is off, whereby, NFET of the BTI circuit 120 is stressed during idle state of the BTI circuit 120. For example, NFET stress causes a shift of voltage (VT) of the NFET and degrade Wordline fall time of BTI circuit 120. As previously described, BTI occurs in transistors that are ON i.e. there is an electric field between Gate and Source OR Gate and Drain, of the transistors of the IC 100. For example, electrons in operating channel of the transistors, are trapped into the gate oxide defects at the silicon-silicon dioxide interface, or deeper in the oxide due to the Gate to Source, Drain electric field of the integrated circuits. This effectively increases the minimum voltage needed at the gate with respect to the source, to turn on the transistor of IC 100. As such, when the NFET is ON (stressed), the electric field exists possibly causing degradation in the VT of the NFET.

However, alternatively, when the NFET is OFF, for instance, it is possible that some of the trapped electrons transmit back to operating channel of IC 100, due to the reverse electric field of the IC 100. Therefore, this there is some recovery of the VT shift possible when the device is OFF. This is known as operating relaxation state of the IC 100. Idle state is the state of the memory to indicate the state when the memory is not accessed and all the circuits are reset to their default or steady state. Signal Y is an output signal of the BTI circuit 120 for affected circuit stress due to BTI degradation events of IC 100 of FIG. 1, whereby, a NFET device of IC 100, is stressed, during idle state of IC 100, according to embodiments.

FIG. 2B illustrates BTI circuit 120 for affected circuit stress BTI degradation events of IC 100 of FIG. 1, whereby, VT stress is reduced on functional NFET, NFET is OFF(VGS=0), during IDLE state of IC 100 and keeper NFET is turned ON, according to embodiments.

For example, in the depicted embodiment, signal B, which is a short pulse generation, gets trigged, based on a rising edge of signal A. Further, during idle period of IC 100, signal B is low to minimize stress on NFET. Keeper device is turned ON to preserve state and degradation of IC 100.

In illustrated depiction, by adding the keeper device on the IC 100, which keeps the idle state of the wordline driver output, pulse B is generated, to effect the output transition at the required time instant which is the same as the trailing transition of signal A gate of the main NFET device to effect the wordline falling transition. This puts the main NFET device in OFF mode during idle period, thus putting it in relaxation mode for most of its life. The keeper exists to only keep the state low on the wordline, it will still perform its logic function. The signal A is used to shut off the NFET device, and is also used to turn ON the keeper. Signal Y is an output signal of BTI circuit 120 for affected circuit stress BTI degradation events of IC 100 of FIG. 1, whereby, VT stress is reduced on functional NFET, NFET is OFF(VGS=0), during IDLE state of IC 100 and keeper NFET is turned ON, according to embodiments.

FIG. 3A illustrates BTI circuit 120 for affected circuit stress BTI degradation events of IC 100 of FIG. 1, whereby, PFET device of IC 100 is stressed, during idle state of IC 100, according to embodiments.

In the illustrated embodiment, input of the circuit, input A is LOW in idle state of IC 100 and high in operation of IC 100 i.e. the PFET would be ON for most of its life, while the NFET would be ON only during the small functional time. The circuit in FIG. 3A illustrates, PFET and NFET degradation during burin and field operation (operating life) show that the degradation for the PFET is higher than for the NFET during burn. As such, there is some more degradation in the field too, but overall, the PFET which is ON for a lot more time than the NFET, degrades more than the NFET. The circuit in FIG. 3A illustrates the problem with a circuit that has the opposite function of the Wordline driver. Signal Y is an output signal of the BTI circuit 120 for affected circuit stress BTI degradation events of IC 100 of FIG. 1, whereby, PFET device of IC 100 is stressed, during idle state of IC 100, according to embodiments.

FIG. 3B illustrates BTI circuit 120 for affected circuit stress BTI degradation events of IC 100 of FIG. 1, whereby, VT stress is reduced on functional PFET by keeping IC 100 OFF(VGS=0), during IDLE state of IC 100, and keeping keeper PFET ON, according to embodiments.

For example, in the depicted embodiment, signal B, which is a short pulse generation, gets trigged, based on a rising edge of signal A. Further, during idle period of IC 100, signal B is high to minimize stress on PFET. Keeper device is turned ON to preserve state and degradation of IC 100. Also, in the illustrated embodiment, by adding the Keeper device to keep the idle state of the wordline driver output), but only pulsing (pulse B is generated—pulse generation circuit is well known and not shown here—to effect the output transition at the required time instant which is the same as the trailing transition of signal A) the gate of the main PFET device to effect the output rising transition. This puts the main PFET device in OFF mode during idle period, thus putting it in relaxation mode for most of its life. The keeper exists to only keep the state low on the output, (not for effecting a transition on the wordline), so a VT shift on the keeper is not important—it will still perform its logic function. The signal A is used to shut off the NFET device, and is also used to turn ON the keeper. Signal Y is an output signal of the BTI circuit 120 for affected circuit stress BTI degradation events of IC 100 of FIG. 1, whereby, VT stress is reduced on functional PFET by keeping IC 100 OFF(VGS=0), during IDLE state of IC 100, and keeping keeper PFET ON, according to embodiments.

The descriptions of the various embodiments of the present invention have been presented for purposes of illustration, but are not intended to be exhaustive or limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The terminology used herein was chosen to best explain the principles of the embodiment, the practical application or technical improvement over technologies found in the marketplace, or to enable others of ordinary skill in the art to understand the embodiments disclosed herein. 

What is claimed is:
 1. A circuit structure, comprising: a first pfet device; a first nfet device connected to the pfet device; a keeper nfet device that reduces stress associated with the first nfet device by keeping the first nfet device off during its functional state; and a keeper pfet device that reduces stress associated with the first pfet device by keeping the first pfet device off during its functional state.
 2. The circuit structure of claim 1, wherein the keeper nfet device is on and active to preserve operating state and degradation of the keeper nfet device.
 3. The circuit structure of claim 1, wherein the keeper pfet device is on and active preserve operating state and degradation of the keeper pfet device.
 4. The circuit structure according to claim 1, wherein the associated first nfet device operates in relaxation mode, once stress of the first nfet is reduced.
 5. The circuit structure according to claim 1, wherein the associated first pfet device operates in relaxation mode, once stress of the first pfet device is reduced.
 6. The circuit structure according to claim 1, wherein stress of the associated first nfet device is based on negative bias temperature instability of the circuit structure.
 7. The circuit structure according to claim 6, wherein the negative bias temperature instability exhibits an increase in non-mobile negative charge during stress of the circuit structure.
 8. The circuit structure according to claim 1, wherein stress of the associated first pfet device is based on negative bias temperature instability of the circuit structure.
 9. The circuit structure according to claim 1, wherein stress of the associated first pfet device is based on positive bias temperature instability of the circuit structure.
 10. The circuit structure according to claim 9, wherein the negative bias temperature instability exhibits an increase in non-mobile positive charge during stress of the circuit structure.
 11. A method for minimizing bias temperature instability degradation of integrated circuits, the method comprising: providing a first pfet device; providing a first nfet device connected to the pfet device; providing a keeper nfet device for reducing stress associated with the first nfet device by keeping the first nfet device off during its functional state; and providing a keeper pfet device for reducing stress associated with the first pfet device by keeping the first pfet device off during its functional state.
 12. The method according to claim 11, wherein the keeper nfet device is on and active to preserve operating state and degradation of the keeper nfet device.
 13. The method according to claim 11, wherein the keeper pfet device is on and active preserve operating state and degradation of the keeper pfet device.
 14. The method according to claim 11, wherein the associated first nfet device operates in relaxation mode, once stress of the first nfet is reduced.
 15. The method according to claim 11, wherein the associated first pfet device operates in relaxation mode, once stress of the first pfet device is reduced.
 16. The method according to claim 11, wherein stress of the associated first nfet device is based on negative bias temperature instability of the circuit structure.
 17. The method according to claim 16, wherein the negative bias temperature instability exhibits an increase in non-mobile negative charge during stress of the circuit structure.
 18. The method according to claim 11, wherein stress of the associated first pfet device is based on negative bias temperature instability of the circuit structure.
 19. The method according to claim 11, wherein stress of the associated first pfet device is based on positive bias temperature instability of the circuit structure.
 20. The method according to claim 19, wherein the negative bias temperature instability exhibits an increase in non-mobile positive charge during stress of the circuit structure. 